1. Field of the Invention
The present invention relates to a fuel assembly and, more particularly, to a fuel assembly suitable for use in a boiling water reactor.
2. Description of the Related Art
The core of such a boiling water reactor is provided with a plurality of fuel assemblies which are spaced apart from one another at predetermined intervals and in which a plurality of control rods which are inserted between adjacent fuel assemblies.
The fuel assembly has an upper tie plate, a lower tie plate and a plurality of fuel rods whose opposite ends are supported by the upper and lower tie plates. Each of the fuel rod includes a multiplicity of fuel pellets. A channel box is mounted on the upper tie plate to surround a fuel bundle.
The maximum power in the core provided with such fuel assemblies is obtained by multiplying the product of the following three kinds of peaking and the average power of the fuel assemblies within the core. A first peaking from among the three kinds of peaking is a radial power peaking which is the proportion of the maximum power of the fuel assemblies within the reactor core to the average power of fuel assemblies. A second peaking is an axial power peaking which is the proportion of the maximum power to the average power of the reactor core in the vertical direction thereof. A third peaking is a local power peaking which is the proportion of the maximum power of the fuel rods in the fuel assembly to the average rods power in the fuel assembly. The power P of each fuel rod in the fuel assembly is given by EQU P=.phi..multidot..delta..sub.r .multidot.N
where .phi. is the thermal neutron flux in the position of the fuel rod, .delta..sub.r is the fission cross section of a fissile material and N is the density of atoms in the fissile material in the fuel rod (hereinafter referred to as "fuel atoms").
In order to efficiently burn the fuel and to prolong its burn-up period, it is necessary to increase the so-called infinite multiplication factor of the fuel assembly. As is known, to increase the infinite multiplication factor, it is effective to increase the density of fuel atoms in a region in which thermal neutron flux level is high, as well as to decrease the density of fuel atoms in a region in which thermal neutron flux level is low. In the above-described boiling water reactor, the thermal neutron flux level is high in the periphery of the fuel assembly, but is low in the central portion due to the non-uniform distribution of a moderator for neutrons, the neutron absorption effect of the fuel rod itself and so forth. Accordingly, it is desirable that a fuel assembly of the type which is used in the boiling water reactor be formed such that the density of fuel atoms in the periphery of the fuel assembly is greater than that in the central portion.
The fuel assembly disclosed in Japanese Patent Unexamined Publication No. 58-26292 is known as a fuel assembly capable of satisfying such a demand.
The fuel assembly disclosed in Japanese Patent Unexamined Publication No. 58-26292 is constructed in the following manner. A plurality of fuel rods for use in a reactor which employs a fissile material as fuel are incorporated in the fuel assembly in parallel to one another and in an integral form. The average density of the fissile material in the fuel rods in the periphery of the fuel assembly is selected so as to be greater than the average density of the fissile material in the fuel rods in the central portion of the fuel assembly. In a manner similar to that disclosed in U.S. Pat. No. 4,229,258, the proportion of fissile material contained in each of the fuel rods is changed in the axial direction so that the infinite multiplication factor in the upper portion of the fuel assembly becomes greater than the infinite multiplication factor in the lower portion of the fuel assembly. Thus, the infinite multiplication factor of the overall fuel assembly is increased and the burn-up period of the fuel assembly is consequently prolonged.
Recently, fuel with a high degree of burn-up has been developed and there has been a trend toward an increase in the enrichment of a fuel pellet of the type which is loaded in a fuel rod.